The conventional method for recovering plasmid from a bacterial lysate after alkaline lysis is commonly known as a bind, wash, and elute process. This process recovers plasmid from a cleared lysate by binding to a glass fiber filter in the presence of a chaotropic agent, such as potassium iodide. This chaotropic agent causes the plasmid to bind to the glass fibers, while most of the other cell constituents pass through. The glass fiber filter is then washed with ethanol (70% or higher by weight) to remove the chaotropic agent. Excess ethanol is removed from the underside of the filter plate by blotting, centrifugation or extensive vacuum drying. The plasmid is then eluted from the glass fibers using water or a low salt buffer.
This method has many drawbacks. First, it introduces a contaminant (ethanol) to the system, which is difficult to fully remove. The residual ethanol may adversely affect the plasmids or the tests performed on them. Additionally, this is a time consuming process and requires many sequential steps. Further, the capacity of the glass fibers to bind the plasmids is limited, making the binding inefficient. Likewise, the elution from the glass is not complete. Some plasmid has been found to irreversibly bind to the glass. In sum, the recovery of the plasmids often is less then 80%, sometimes less then 70% of the available plasmids.
The present invention provides a better method and apparatus for recovering plasmids or other circular DNA which eliminates the introduction of new contaminants, provides for higher plasmid recovery rates with higher purity and which is much faster than the current process.